Alpha-numeric and graphic displays comprise a variety of methods and apparatus which typically fall within two general categories, e.g., dot matrix type displays and vector or stroke type displays.
Dot matrix type displays form alpha-numeric characters and graphic images by selectively illuminating, printing or otherwise making visible dots or pixels on a CRT screen, flat panel display, paper, or other recording medium. Depending on the density of the dots or pixels, an alpha-numeric character or graphic image on a dot matrix type display can appear, to an unaided eye, as a plurality of line segments. In contrast, an alpha-numeric character or graphic image on a vector or stroke type display does, in fact, comprise a plurality of line segments.
A stroke on a stroke type display, such as a CRT or pen recorder, is usually generated using analog signals, typically ramps, applied to X and Y controls. For example, in an electrostatic deflection CRT type stroke display, an electron beam is displaced from a zero potential position by the application of positive and negative potentials to X and Y electrodes. If the X electrode controls horizontal movement of the beam and the Y electrode controls vertical movement of the beam, a changing potential applied simultaneously to both the X and Y electrodes will produce a stroke on the CRT which has both an X component and a Y component. The magnitude and the polarity of the change of the X and Y potentials applied to the X and Y electrodes determine the length and slope of the stroke.
In a magnetic deflection CRT type stroke display an electron beam is deflected in an X and Y direction by the application of currents to the X and Y windings of a deflection yoke. Typically, the currents are provided by a voltage controlled current source coupled to the yoke.
In addition to the magnitude and polarity of the potentials applied to the X and Y electrodes in the electrostatic deflection CRT type stroke displays and the currents applied to the X and Y windings of the yoke in the magnetic deflection CRT type stroke displays, as described above, the rate of change of the potentials and currents applied to the X and Y electrodes and X and Y windings must also be taken into consideration in order to control the brightness or intensity of the stroke over its length. This is because a fast-moving electron beam over a given distance will produce a stroke of lesser intensity than a slower moving beam over the same distance. As a consequence of this phenomenon, CRT type stroke displays can use beam velocity to control intensity.
Heretofore, several proposals have been made for providing for beam movement. For example, stroke type displays with means for providing beam movement have included digital-to-analog converters (DAC) in combination with rate generators for generating the required deflection potentials, such as shown and described in U.S. Pat. No. 3,800,183 and U.S. Pat. No. 4,369,441. While providing for high resolution displays, a principal disadvantage of the prior known apparatus is that the use of a DAC is relatively expensive and generally requires a considerable amount of space.
In another U.S. patent, U.S. Pat. No. 4,032,768, there is shown and described a stroke type display in which simultaneous .DELTA.X and .DELTA.Y step voltages are converted to variable rate ramp voltage pairs. A disadvantage of this display apparatus is that it requires the use of the non-linear properties of well-matched transistors.
The above discussion of CRT stroke type displays also applies to other types of stroke type displays, such as, for example, pen recorders.
Pen recorders also employ X and Y controls for controlling the movement of a pen along X and Y coordinates.